Recently, a direct-type backlight unit using light emitting diodes (LEDs) is capable of local dimming, has high luminance uniformity and no mercury pollution problem, such that the direct-type backlight unit becomes popular. The direct-type backlight unit can be implemented by two conventional methods. In the first method, a surface mount type LED is adopted, but this method normally requires a large number of LEDs. On the other hand, in the second method, a surface mount type LED and a second lens are used together, which can reduce the number of LEDs used. However, no matter what method is used, it requires a support structure to support the diffusion plate for preventing deformation of the diffusion plate caused by thermal expansion, handling and transportation, which may reduce optical performance and damage LEDs if the height of a cavity accommodating the diffusion plate and the LEDs is low.
Please refer to FIG. 1, it is a structure view of a conventional direct-type backlight unit. The backlight unit comprises: an optical sheet 940, a diffusion plate 960, a bottom plate 910, a plurality of LEDs, and a plurality of support structures 930. For convenience of explanation, FIG. 1 only shows two LEDs 920 and a support structure 930.
The optical sheet 940 is disposed on the diffusion plate 960; the bottom plate 910 is disposed below the diffusion plate 960; the LEDs 920 are disposed between the diffusion plate 960 and the bottom plate 910; the support structure 930 is disposed between the diffusion plate 960 and the bottom plate 910, and one end of the support structure 930 is fixed on the bottom plate 910 and the other end of the support structure 930 is contacted with the diffusion plate 960.
Please refer to FIG. 3, it is a luminous intensity distribution curve of the LED 920 with an additional secondary lens. The black line 8 in the FIG. 3 shows an LED having bat-type light field with different luminous intensities in different angles. In addition, from FIG. 3, the light intensity in the two lateral sides is stronger than that of the middle portion in the LED.
Please see FIG. 2, when the LEDs 920 emit light, the light is emitted into a bat shape. However, the light is blocked by the support structure 930 and can't pass through, resulting in a dark area 950 on the diffusion plate 960.
Therefore, it is required to provide a novel backlight unit and a crystal display device using the same in order to solve the dark area problem on the diffusion plate.